1. Field of the Invention
The present invention relates to systems and methods for stabilizing lasers, and CO2 lasers in particular.
2. Description of the Prior Art
A CO2 laser uses gaseous CO2 as the gain medium. The laser beam is formed by the energy state transitions between vibrational and rotational states of the CO2 molecules that emit radiation at infrared wavelengths between 9 and 11 microns, and typically at 10.6 microns. At this wavelength, many materials such as glass, plastics, water, and certain types of silicon (e.g. doped silicon) are relatively opaque.
In a common form of a CO2 laser, the CO2 gain medium resides between two electrodes and is excited by a radio-frequency (RF) oscillator to generate a plasma. The RF excitation is modulated at a frequency of about 120 kHz, with a duty cycle ranging anywhere from about 18% to about 88%. The modulation is used to prevent arcing, i.e., the formation of a preferred electrical path through the RF-generated plasma, which results in an electrical short between the electrodes. The modulation also provides for relaxation time for the gain medium to recharge. Unfortunately, the modulation requirement for RF-excited CO2 lasers makes direct analog feedback control impossible.
Commercial CO2 lasers are available from a number of manufacturers (e.g., Coherent, Inc., Santa Clara, Calif.), and can have very high continuous output power to levels (e.g., 300 W from the commercially available DIAMOND™-K-300 from Coherent, Inc.). Specially constructed CO2 lasers are capable of generating tens of Megawatts of continuous wave output power.
CO2 lasers are capable of producing a very high output power relative to other types of lasers because of their relatively high efficiency. The typical efficiency for a CO2 laser (measured as the ratio of input electrical power to output optical power) typically ranges from about 5 to 20 percent, which is about 100× greater than that for the more common types of lasers, such as helium-neon, argon-ion or krypton-ion lasers.
Because of their high power and IR wavelength output, CO2 lasers have found wide applications in industry, from medical applications to semiconductor processing, to welding and cutting operations.
One recently developed application for CO2 lasers is laser thermal annealing or “LTA” (also referred to more generally as laser thermal processing or “LTP”) of semiconductor substrates in semiconductor manufacturing. The LTP process is described in detail in U.S. Pat. No. 6,747,245.
A key requirement for LTP is that the laser heating be relatively uniform over the wafer being processed. For example, when performing LTP for the non-melt annealing of junctions on a silicon wafer, the maximum annealing temperature seen by any point on the wafer must be within about ±10° C., and preferably within ±5° C. This requires a laser power stability (i.e., variation in power vs. time) of about 0.35%. Unfortunately, commercially available CO2 lasers exhibit a stability of about ±8%, which translates into a temperature variation during annealing of about ±100° C. at the required annealing temperate of 1,300° C.
It is anticipated that other industrial applications using CO2 lasers will be developed that will require, or that would benefit from, a greater degree of stability in the output beam power or from feedback control of the beam power from a substrate temperature measurement system.